I'ts quite simple, Ian.
Let's assume you have that switch operating a space heater, or other heavy load. The switch simply won't last very long, and stop being a switch. There might not even be any fireworks; the contacts just won't close anymore, and you get to replace the switch.
That's the whole point to the 'motor rating' idea. Motors, and some other loads, require a lot more than the rated current under specific circumstances .... so the switch will need to be able to handle those moments.
Once again, we come up against an example where 'trade practice' has been decades ahead of 'code.' In this instance, I'm referring to the trade proctice of separating the lighting circuits from the receptacle circuits, and using 15 amp circuits for lighting.
